Molecular beam tube



June 27, 1967 Filed March 5, 1963 2 Sheets-Sheet 1 A 2 X vb 7 1 40 FIG. I g- T Q 8 AW, Mognetic Field Intensity =0 Frequerjcy Signal gg i Master g dgzg; Output Power Oscillmo' Conirol v Amplifier 1 FIG. 2 v I Modulation Oscillator 50 380 366 384 3 356 370 "All B Mognel 352' 300 354 M q 202 250 F= Om =O F=1m,=01o F=1m =-1 F=Om =0 v 3 "$0 I? F== Om w W '4 M T FIG 1 F=1mF=+1 F 1m O INVENTOR JAM-ES GEORGE BY 4 hvi ATTORNEYS June 27, 1967 J. GEORGE 2 MOLECULAR BEAM TUBE Filed March a, 1963 2 Sheets-Sheet 2 FIG. 4

1N VEN TOR.

JAMES GEORGE ATTORNEYS United States Patent M 3,328,633 MOLECULAR BEAM TUBE James George, Swampscott, Mass., assignor to National Company, Inc., Malden, Mass., a corporation of Massachusetts Filed Mar. 5, 1963, Ser. No. 263,005 19 Claims. (Cl. 315111) My invention relates to an apparatus for providing an accurate frequency standard by employing a spectral line of thallium as a reference frequency. More particularly my invention concerns a thallium beam apparatus employing a thallium source and utilizing a resonance frequency of the thallium atom as a frequency standard.

Molecular or atomic frequency standards commonly utilize as a reference frequency the substantially invariant frequency corresponding to the transition of a molecule or atom from one energy state to another energy state.

(As used herein and in the claims molecule and molecular will be used to refer both to atoms and molecules, without distinction.) A molecular beam apparatus employing the resonance frequency of cesium as it changes from one energy state to another as a frequency standard is described in US. Patent No. 2,972,115 issued to J. R. Zacharias et al. dated Feb. 14, 1961. There is of course a continuing need for developing frequency standards of greater accuracy, reliability, stability and operational life. Although cesium as a molecular beam source and standard has found acceptance, the utilization of cesium has inherent operational characteristics which contribute to some inaccuracy, some unreliability and which limit operational life.

A conventional cesium molecular beam apparatus comprises a source chamber from which molecular cesium in vapor form effuses, and a collimator which forms the eifused vapor into a narrow beam and directs it through a long evacuated cylindrical beam tube. While passing through the cylinder from the one end to the other end the molecular beam is subjected to steady magnetic fields A and B which have a strong field gradient transverse to the beam direction which fields deflect molecules according to their energy states. Intermediate these fields the molecular beam is subjected to a steady Weak uniform C magnetic field. While in this region the cesium molecular beam is also passed through a radio frequency (RF) cavity wherein the beam is subjected to an oscillating externally generated field of approximately the resonance frequency required for the molecules to satisfy the energy difference relationship and of sufiicient intensity to effect energy transitions. To extract the desired signal a portion of the neutral cesium molecular beam emerging from the B magnetic field which has undergone energy transitions is passed through an aperture and converted into an electric current proportional to the flux of the particles by a detector. This detector permits the beam to impinge on a hot wire at the other end ofthe beam tube thereby ionizing the molecules and allowing them to subsequently strike the input plate of an"electrostatic electron multiplier. A highly accurate cesium frequency standard is obtained in this manner, which may be compared.

to an externally generated frequency.

As compared to thallium the cesium molecular beam standard is relatively sensitive to magnetic C field inhomogeneity. The change is frequency (Af) of cesium and thallium with a change in the intensity (H) of C field may be represented as follows:

.For cesium: Af=93.9 10- HAH For thallium: Af=1.92 10- HAH Thallium is then about fifty times less sensitive to C field inhomogeneity than cesium and thus permits a'correspond- 3,328,635 Patented June 27, 1967 ing enhancement in the accuracy of the frequency standard.

Further, a cesium source has a vapor pressure of about 1 l0- millimeters of mercury at 25 C., the common ambient operating temperature of the cesium molecular beam apparatus. Because of this'relatively high vapor pressure the employment of a cesium getter is usually required to inhibit undesirable cesium background flux. Cesium vapor from the undesired and deflected cesium atoms in the molecular beam apparatus and particularly that cesium vapor located between the hot cesium oven source and the A magnet gives rise to an undesirable cesium background fiux which scatters the cesium beam and increases the noise to signal ratio at the cesium detector. Thallium with a vapor pressure of less than 1 10 millimeters of mercury at a temperature of 25 C. obviates the need for a thallium getter and more readily permits maintenance of a vacuum condition of l- 10-' millimeters of mercury or less in the molecular beam tube. Since a getter is not normally required for a thallium molecular beam tube a source of unreliability in detector current is therefore removed.

Cesium in a magnetic field has sixteen energy states; thallium has only four energy states. The .ratio of molecules undergoing desired useful energy transitions as compared to undesired energy transitions for thallium is then,

about 1 in 4; for cesium it is l in 16-. Thus a thallium beam provides a useful signal about four times greater than a cesium beam for the same beamintensity. This increases the ability of the thallium tube to stay in lock at the resonance frequency thereby improving the operation of the thallium tube.

From the foregoing it is apparent that there are numerous advantages in using thallium as the molecular beam material in a molecular beam frequency standard. However no commercially successful thallium beam tube has yet been developed. The diificulties of developing a thallium molecular beam tube are associated with the need for an improved and modified oven source, a suitable waveguide cavity and a long life thallium detector all integrated to provide an operable successful molecular beam frequency standard of enhanced accuracy, reliability and stability.

It is therefore an object of my invention to provide an improved thallium molecular beam tube apparatus of enhanced accuracy, reliability, stability and operational life. Another object is to provide an improved molecular beam tube apparatus of the type describedemploying a thallium source'which beam tube apparatus is relatively insensitive to.C field inhomogeneity. A further object of my invention is to provide an improved thallium oven source and collimator for use in molecular beam devices.

Still anotherobject of my invention is to develop an improved double cavitywaveguide structure for use with a molecular beam tube apparatus. A still further object of my invention is to provide an improved ionizer characterized by improved ionizing surface which ionizer is particularly useful with thallium.

. .Further objects of my invention will also be apparent to those skilled in the art from the following description of my invention taken in conjunction with the accompanyin-g drawings in which:

5 FIG. 1 is a graphical representation of the various energy levels of thallium 205 in a magnetic field'of varying intensity;

FIG. 2 is a drawing, partially pictorial and partially in block and line form of a thallium molecular beam tube apparatus;

FIG. 3 is a general diagrammatic representation of the passage of a thallium atom beam through the thallium molecular beam apparatus of FIGURE 2;'

FIG. 4 is an axially cross-sectional view through the thallium oven source and collimator of FIGURE 2 made in accordance with my invention;

FIG. 5 is an isometric view of one end of a modified double cavity microwave waveguide structure made in accordance with my invention;

FIG. 6 is an axially cross-sectional view of the improved ionizer in FIGURE 2 particularly useful with thallium, and;

FIG. 7 is a cross-sectional view of an ionizer tube of the ionizer taken along the lines 7-7 of FIGURE 6.

FIGURE 1 is a graphical representation of the energy states of a thallium atom in a field free and a magnetic field condition. The total angular momentum of the thallium atom (F) is the sum of the nuclear moment (I) i the electron moments (I In a field free condition I is /2 and J is 1 /2 giving rise to two energy states, F:1 and F :0 with an energy difference (AW as shown on the abscissa line of the graph. However in a magnetic field the thallium atom energy is quantized in four separate energy state srepresented by the total angular momentum (F) and the total magnetic momentum (m Thus in a magnetic field thallium can exist as F :Om :O; lm :l', F: lm :O and F: lm +1. In a strong inhomogeneous magentic field, such as provided by A and B energy state selectors to be hereinafter described, the spatial separation of the thallium 205 molecules into various energy states occurs. In a weak magnetic field such as provided by the uniform C field an energy transition is effected by an oscillating field within a RF cavity. The energy transition may be effected from the higher to the lower state with the radiation of the excess energy or from the lower to the higher state with the absorption of the necessary energy. In the embodiment described the energy transition (AW as shown in FIGURE 1) occurs from the higher energy state (F: 1m :0) to the lower energy state (F:Om :0). Though clearly not the only transition posible this m =O state for transitions is preferred, because it is not linearly dependent on the C field strength. The frequency of the wave involved in such transitions is determined by f:1/h (AW) wherein f is the wave frequency and h is a universal constant.

T hallium beam; apparatus and operation FIG. 2 is a representation of a thallium molecular beam frequency standard made according to my invention which comprises in combination; a long cylindrical evacuated beam tube 50 having at one end thereof a thallium beam source such as a thallium oven 100 in a heat exchange relationship with a collimator 150 to form the beam of molecular thallium into a predetermined geometric configuration and to direct the beam through the evacuated beam tube. Externally mounted about the tube are a first energy and second energy state selectors (H and (H respectively, such as strong inhomogeneous magnets 10 and 20. Intermediate these energy state selectors is a doubel cavity waveguide structure 350 having resonant cavities 352 and 354 axially aligned with the beam tube. This structure is placed within weak uniform C magnetic field (H contained within a C field shield 300. The A, B and C fields are disposed to provide continuity between each field and to avoid field free conditions in passing from the A to C and C to B fields to avoid loss of coherency in energy state selection. A convolution is provided in the beam tube for adjusting the tube geometry to select the proper energy transition. A thallium ionizer 200 having a source of oxygen 250 converts the selected neutral beam into thallium ions. The thallium ions are passed through a mass spectrometer 40 to separate interfering ions and to an electron multiplier to convert the ion flux into a proportional electric signal. The signal from the electron multiplier 30 derived from the ionized thallium beam is then fed to the conventional circuit shown such as that disclosed in the Zacharias et al patent noted above or in US. Patent No. 2,906,663 issued Nov. 15, 1960, to W. A. Mainberger.

The A and B energy state selectors are C-shaped permanent magnets. In this arrangement the field lines of each magnet from a large area concave pole piece on one side of the beam coverage on a small area convex pole piece on the opposite side of the beam, and are transverse to the thallium molecular beam. The path length of the A and B magnetic field is about three times as long as that for the A and B magnets used in a cesium beam tube frequency standard since the magnetic moment of thallium 205 is only one third that of cesium 133. I have found for example that the A magnet is about three inches long.

FIG. 3 represents the trajectory of the thallium beam as it proceeds through the thallium molecular beam apparatus in FIG. 2. Thus in the thallium beam source 100, elemental thallium is converted into a molecular beam of predetermined geometry and thermal velocity. This beam is then subjected to a strong inhomogeneous magnetic field by a magnet 10 which provides the field H In the magnet field H the thallium beam is quantized as shown in FIG. 1 to various energy states. The inhomogeneous field has a predetermined intensity gradient across the opposing pole piece to create a deflecting force. With thallium a deflecting force or field gradient of at least 10 kilogauss per centimeter or higher is required to spatially separate the thallium beam. The energy state selector field must also have a mean strong field flux of at least about 14 kilogauss/cm In this manner the higher energy state molecules will move toward the lower end of the gradient and the higher energy state toward the lower end.

Of course, as in the cesium molecular beam devices, the entire beam of thallium atoms should be axially disposed to about the same equal potential within this field gradient. With the thallium molecular beam in the A energy state selector there is a strong magnetic field with a predetermined intensity and gradient and therefore there is a spatial separation of the energy state of the thallium atom with the two of the four energy states being deflected as diagrammatically shown in FIG. 3. By controlling the height and width of the thallium beam dimensions the thallium beam may be placed in an equipotential portion of the field I-I so that all the thallium atoms receive the same deflecting force and that substantially the same number of nuclear and electron dipoles are induced over the area of the beam. As the beam emerges from the A magnet field the tube geometry is so arranged to pick either one of the energy states. In the present example the collimator and the tube are arranged to permit the higher two energy states to pass up the axial longitudinal length of the beam tube.

If all the atoms of thallium in a single energy state were to be selected, the tube deflection would have to be quite large. This comes about because the thallium atoms have a wide distribution of thermal velocities as they enter the field H and accordingly the deflecting force of the field acts on them for different time periods. To reduce this tube deflection, a particular energy distribution of atom velocities which is reasonably and significantly separated from the optical beam path is picked. The low velocity thallium atoms have a low intensity i.e. there are relatively few of them in the thallium beam, and a relatively large deflection angle. The high velocity atoms have such a relatively small deflection angle that the atoms form a part of the optical beam and are not used in the resonance frequency measurement. I therefore make a compromise between the intensity and the deflection of the thallium atoms to obtain a significant number of thallium atoms for use in measuring their resonance frequency. Commonly those atoms having a distribution of' 0.5 to 1.0 are selected for use in the beam tube.

In the example shown in FIG. 3, after the A magnet field has selected the higher energy states F: 1, m :+1 and F :1, m :0, the thallium molecular beam passes through a weak magnetic field H This field is provided to avoid field free conditions and thereby the loss of the coherence in the initial energy state selection of the thallium atoms. In the weak homogeneous magnetic field H the selected thallium beam is subjected to a magnetic strength of l gauss or less and preferably of the order of 50 to 100 milligauss.

The relatively Weak uniform magnetic field must be homogeneous, since if the field varies the frequency of precession of the thallium changes. As discussed above the cesium atom is particularly sensitive to variations in the C field homogeneity, while the thallium atom is relatively insensitive to variations in this field.

In magnetic field H the atomic thallium beam enters a first resonant cavity in the double resonant cavity waveguide structure 300. In this first cavity the thallium beam encounters an oscillating externally generated elect-romagnetic field e.g. microwave energy from an oscillator that has a frequency approximately equal to the molecular resonance frequency corresponding to the difference in the energy levels of the thallium atom. The weak magnetic field H parallel to A and B field and perpendicular to the beam path aligns the nuclear magnetic field of the thallium atom in the same direction as the weak field. The oscillating electromagnetic energy of a preselected frequency and intensity is fed to the thallium atom in the F=1 m =O state as it passes through the first and second cavities to eifect the desired transition of the thallium atom from the higher energy state F: 1, mF=0 to the lower energy state F=0 m =0. The magnetic vector of the electromagnetic field supplied to the cavity is applied perpendicular to the nuclear magnetic field of the thallium atom and parallel to the weak magnetic field H Upon leaving the first cavity the thallium electrons pre-' cess in phase with the oscillating field. The molecular beam then traverses the weak intervening magnetic field H region called a drift region in which it is not in a field free condition, is undisturbed by outside forces, and the electrons precesses at their natural frequency rate. The thallium beam upon entering the second resonant cavity is again subjected to the oscillatory electromagnetic field. The electrical length between cavities should be the same from the center feed point of the waveguide structure to have the thallium in both cavities in the same phase. In the second resonant cavity if the frequency ofthe oscillating electromagnetic energy is the same as the atomic resonance frequency, i.e. the natural precession rate of the electrons, and has not varied from that of the first cavity, the oscillatory radiation will be in phase and the molecules will go to the lower energy state by yielding up wave energy in the form of a photon. Any phase dif-, ference between the first and second cavities will depend upon the difference in frequency of the oscillating electromagnetic field and the natural frequency of precession ofthe electrons and the velocity of the atom i.e. the time that the atom takes to traverse the distance between the two cavities, or the time the beam spends in the drift space. Thus when emerging from the second resonant cavity and the weak C field H a certain amount of the molecules have undergone an energy state transition from F=1 m =0 to F=0 m =0.

The molecular thallium beam then enters a second energy state selector such as a B magnet which is constructed like the A magnet and which separates the thallium beam into two energy states. Those molecules which failed to undergo the desired transition, that is those atoms in the F: 1, m =0 state are deflected toward the higher or strong portion of the B field gradient and are removed from the beam path. The remainder of the molecular beam which is composed of those molecules of the lower energy state F: 0, m =0, is permitted to pass to thallium ionizer 200 at the other end of the beam tube 50.

In the thallium detector, which preferably includes a hot wire ionizer or other thallium detectingmeans, the neutral thallium molecules are converted into an electrical atoms as they are deflected in a strong magnetic field, the

ionized thallium atoms may be separated from the lighter or heavier mass impurities such as potassium ions with the purified thallium ion beam being directed to the input plate of an electron multiplier 30 for conversion and amplification into a direct current signal. The output of the electron multiplier 30 is then fed to an oscillator frequency control circuit of any conventional type which adjusts the frequency of the master oscillator, to the frequency synthesizer and power amplifier from which microwave energy is fed to the waveguide structure 300. The signal output from the frequency synthesizer then represents the natural resonance frequency of the thallium in effecting the desired transition between energy states. This natural resonance frequency of thallium 205 is 21,310.8351-(10005 mc./s. The frequency synthesizer is phase modulated by a. modulation oscillator between about 1 to cycles per second; the modulation signal is also supplied to the oscillator frequency control circuit as a reference signal for phase comparison purposes. Frequency control apparatus which might be used with the improved beam tube of my invention is described more fully in US. Patent No. 2,960,663 to W. A. Mainberger issued Nov. 15, 1960.

T hallium beam source FIG. 4 shows a thallium source and collimator generally indicated at 100 and 150 respectively which comprises in combination: an outer jacket 102 and an inner jacket 104 disposed within the outer jacket 102 and securely fastened thereto in a vacuum tight manner. The inner jacket 104 is secured at the closed end of the outer jacket to a ceramic mounting block 108 and at the other end is welded to the outer'po-rtion of the inner jacket for ease of fabrication. The outer jacket is provided with a scalable vacuum pump outlet 110 which permits a vacuum pump (not shown) tocreate a heat insulating subatmospheric or-vacuum condition between the outer and inner jacket. The-vacuum region between the inner and outer jackets preferably being a vacuum of the orderof 1X 10- mm.

of mercury'functions as an efilcient heat insulator. A thallium oven 114 is axially disposed within the inner jacket 104 and is securely connected to the ceramic mounting 108 at one end and to a collimator 150 at the other end. The oven 114 contains a source of pure elemental thallium 205 metal 116. Electric heating elements 118 are externally mounted about and around the external periphery of the thallium oven 114, and at least a portion of the collimator 150. Electrical leads 120 pass through glass to metal seals in the oven header 121 to supply power to the element 118. Elongated longitudinal tubular beam pipes 122 and 124 having an internal diameter of approximately 0.005 inch are axially disposed within the oven and slight- 1y above the thallium source material 116 and closely adjacent to one end of the collimator 150. These pipes function as battles to prevent the molten thallium from entering the collimator and serve as an effusion aperture for the thallium molecules by directing the molecules into one end of the collimator means 150'. V

The collimator 150 comprises a plurality of elongated longitudinally [disposed channels or canals. Its purpose is to give the beam of thallium molceules a predetermined shape and direction such as a rectangular, square, circular, or even ellipical form. To this end, it is normally formed from sheets of corrugated metal foil sandwiched between flat sheets as shown, thereby forming a plurality of elongated longitudinal channels which permit the passage of the thallium molecules therethrough. The collimator channels are normally about /2 inch long and have an internal diameter of 20 to 180 thousandths of an inch. As shown the internal heater element 118 is placed about the periphery of both the thallium oven and collimator in a heat exchange relationship with the thallium molecules passing through the collimator pipes. Due to the relatively ly high melting point of thallium, that is 303.5 C., the coils of the heating element should be disposed about the thallium oven and usually at least a portion of the collimator. By this arrangement the thallium vapor is permitted to etfuse from the beam pipes and through the collimator and into the strong magnetic field H without condensing and plugging up the collimator channels. This heat exchange relationship between the heating means and the collimator is not necessary in conventional cesium beam tube devices because of the relatively low melting point of the cesium.

In the embodiment illustrated the internal heating coils supply heat directly to a major portion of the collimator 150, with the remainder of the collimator being heated sufiiciently by thermal radiation. In some embodiments it may be advantageous to have the heating coils completely encircle the collimator and even slightly beyond the farther end to ensure that the thallium does not condense within the collimatin-g channels. As thus shown the heating coil heats the thallium source material to the melting point. A thermal gradient should be maintained between the collimator and the oven with the higher end of the thermal gradient at the farther end of the collimator; that is, the end farthest removed from the thallium oven. A thermal gradient from the one end to the farther end of the collimator of from to 50 C. or higher than the thallium melting point is suitable.

Engaged with the inner jacket and axially aligned with the collimator is an elongated convoluted beam tube guide 126 for adjusting beam tube geometry and a ceramic thermal insulator 124 to inhibit heat loss to the A magnet pole pieces and other parts of the tube. The beam tube guide 126 is so disposed as to be relatively flexible at the bending point 128 to enable the beam tube to form the proper predetermined relationship and axial path between the collimator 150 and the ionizer 200. Due to the corrosive effects of thallium on many metals it is preferred that the thallium oven be composed of stainless steel, such as stainless steel 304. The ceramic base 108 securing the thallium oven 114 to the jacket 104 contains openings 130 to permit the rapid evacuation of trapped air in the thallium oven when the molecular beam tube is evacuated to 1 10"" mm. of mercury or lower vacuum conditions by an ion vacuum pump (not shown). By the apparatus thus described a thallium molecular beam of predetermined geometric configuration is formed and introduced into the strong A magnet field at a particular beam path angle.

The waveguide FIGS. 2 and 5 show a rectangular C-field shield 300 surrounding the central portion of the evacuated beam tube 50. The C-field shield has disposed therein a U- shaped k band waveguide structure 350 having a center microwave feed point 356 equidistant between the centers of a first resonant cavity 352 and a second similar cavity 354. The H, field is created within the shield 300 by longitudinal windings not shown as in the Zacharias et al. patent. A ceramic window 366 is located above the feed point; the window is slotted at its lower end to permit adjustment in the electrical length of each leg of the microwave structure 350.

The thallium beam emerging from the magnet field H is directed through the first and second cylindrical cavities 352 and 354 where it is subject to an oscillating electromagnetic field of predetermined frequency and intensity,

the magnetic vector of this field is parallel to the H field and perpendicular to the thallium beam.

In each resonant cavity the thallium beam is subjected to a frequency of approximately 21,310 megacycles at a power level of approximately 25 to 30 microwatts. A drift space 370 is located between the first and second cavities through which the thalliumbeam is projected after passing from the first and before entering the second cavity. The excitation within each cavity is effected by a common source. Each cavity is characterized by a beam entrance window 372 and 374 and a beam exit window 376 and 378 respectively. Within the rectangular C field shield 300 the entrance window 372 of the first cavity and the exit window 378 of the second cavity are supported by parallel shield end plates 380 aind 386 composed of high magnetic saturation material such as Hypernik to cut oif the strong A and B magnetic fields and end shields 382 and 384 composed of low magnetic saturated material such as Mu metal to act as a magnetic shield.

Of course a single resonant cavity can also be employed, but the described double resonant cavity structure is preferred to lessen sensitivity of the thallium beam tube to variation in C field intensity.

In the microwave apparatus the beam windows of each cavity comprise a honeycomb structure consisting of a plurality of tubes or channels 290 longitudinally disposed along and about the axis of the hallium beam, which tubes or channels project in a tubular, rectangular cylindrical or other predetermined geometric shape from each end of the resonant cavities. I have found that these beam windows may have a total diameter of of an inch and be composed of a plurality of relatively small diameter short length tubes having a length of of an inch and an internal diameter of of an inch. The function of these honeycomb beam windows at each end of the internal cavity is to inhibit the RF oscillating field within the cavity from being propagated through, or entering, the drift space 370 without distorting the desired oscillating field within the cavity. These beam windows project from each cavity and have the screened end of the windows in contact with the internal cavity flush with the internal wall therof to prevent the creation of undesired resonant modes within the cavity.

The resonant cavities can have the same or different or any desired geometry form such as cylindrical as shown or rectangular. The cavities must be axially aligned in the thallium molecular beam path and the oscillating field must be perpendicular to the thallium beam path to inhibit Doppler effects.

In the embodiment of the thallium beam tube apparatus illustrated two cavities are provided having cylindrical windows which cavities are designed to operate in the transverse magnetic TM mode. If the square of the ratio of the cavity diameter to the length of the cavity is less than 1,, the electromagnetic field in the cavity is in a degeneracy region. If this ratio is less than about 0.5, multiple resonant modes can exist within the cavity. In my invention I avoid multiple modes by increasing the cavity length relative to the cavity diameter so that the cavity is resonant between unwanted modes in the degeneracy region. The increased length of the cavity also reduces the radio frequency power requirements to acceptacle levels, since the amount of power required to eifect the transitions depend on the time the thallium molecules are in the cavities.

Thus the resonant cavities are of a shape and dimension to resonate in the TM mode and at least one and preferably two have a grid-like beam window at each cavity end. The cavity length is selected to fall between regions of mode degeneracy and to reduce the radio frequency power requirements to acceptable and economic levels. Further, the creation of unwanted modes within the cavity itself due to resonant cavity discontinuities, can be prevented by a fine mesh screen 377 disposed across 9 the internal end of the cavity exit and entrance beam windows.

The thallium ionizer The detector of the thallium beam tube apparatus comprises in combination an ionizing means 200 to convert the beam of neutral thallium atoms to charged ions, a mass spectrometer 40 to separate the positively charged thallium ions from other ions having higher or lower mass, and an electron multiplier 30 to convert the separated beam of positively charged thallium ions into a direct current signal to be fed to the oscillator frequency control circuit. 7

As seen in FIGURE 2, the ionizer 200 is positioned at the right hand end of the beam tube 50 within the sealed portion of the tube. I prefer to use a hot wire ionization detector in which a neutral beam of atoms impinging on the ionizing contact surface or the hot wire surface are adsorbed and then quickly repulsed or re-evaporated as charged particles or ions. The material employed as the ionizing contact surface and its required surface temperature depend upon the ionization potential of the neutral atoms which are to be ionized and the rate of re-evaporation desired. For example, thallium has an ionization potential of about 6.1 electron volts, which requires the heated ionizing surface to have a work function of about 6.2 electron volts or higher. Although compounds and materials are available having work functions up to about 9.0 electron volts, high work function materials and compounds having a high melting point and thermal characteristics sufficient to withstand long periods at elevated temperatures are difficult to obtain and their ionizing surface rapidly deteriorates. In one embodiment of this invention an apparatus for and a method of maintaining a relatively constant ionizing surface and thereby a relatively constant work function is provided by the continual and automatic maintenance of the desired chemical compound of high work function on the ionizing surface. By this method and apparatus a workable thallium beam tube apparatus of enhanced operational life is made possible.

The ionization of the neutral thallium atomic beam is accomplished by the ionizer 200 illustrated more particularly in FIGURES 6 and 7. It comprises, in combination: a pressurized source of oxygen gas 250 in gas flow communication through a conduit 203 and an adjustable needle valve 206 to control the gas flow to the hollow internal section 204 of an elongated hollow tungsten cylinder 202 the end opposite the gas entrance being closed. The tungsten cylinder is preferably disposed so that its longitudinal axis is substantially at right angles to the axis of the neutral thallium beam. The tungsten cylinder has an elongated slot 203 cut in the outer surface of the cylin- I der. The slot is axially disposed along the external cylin-' der wall. The internal face of the slot 210 (FIGURE 7) constitutes the heated ionizing contact surface upon which the neutral thallium beam impinges. The ionizing surface 210 has a relatively thin continuous surface oxide film 212 of tungsten oxide comprising W W0 or a combination thereof or of other metal oxides. The depth of the slot 210 is selected to create a predetermined degree of relatively uniform wall thickness between the tungsten oxide thin film 212 and the internal wall surface of the tungsten cylinder 202. This Wall should be sufficiently thin to allow the slow diffusion of the pressurized oxygen through the wall of the slotted section of the cylinder e.g. from about 0.010 to 0.030 inch.

The remaining relatively thick Walled tubular section inhibits oxygen diffusion through this wall section and serves to provide structural strength to the ionizing surface. The metal tungsten cylinder or tubular element 202 and the ionizing surface and film 210 and 212 respectively are heated and maintained at an elevated temperature of 600 C. or higher by electrical energy received from wires 214 and 216 attached to each end of the cylinder and passing through the ceramic supporting material 10 218 and 220. The metal cylinder functions as a resistant heating means in an electrical circuit. A pair of negatively charged ion acceleration plates 222 and 224 are also supported in a parallel spaced relationship on the ceramic supports 218 and 220. These plates are provided with apertures 226 and 228 through which the beam of neutral thallium molecules passes to strike the ionizing surface 210 and through which the positively charged thallium ions are accelerated into the magnetic field ofthe mass spectrometer 40. The purpose of these plates is to create an electrostatic field which urges the re-evaporated positive charged thallium 205 ions into the magnetic field of the mass spectrometer 40. Spatially separated from, but supported by the same ceramic plates and the ion acceleration plates 222 and 224 is a parallel disposed positively charge plate 230 which plate is in electrical communication through electrical conduit 232 and an electrical source a to acquire a positive charge. The negative plates are grounded either externally or internally and have 25 to 30 volts between the plates 222 and 224. I

In the operation of the ionizer 200, the neutral beam of thallium atoms impinges on the heated exposed tung sten oxide ionizing surface 212 of the metal cylinder 202. The heated tungsten oxide film surface 212, having a work function higher than the ionization potential of the neutral thallium atom, converts the neutral thallium atoms into positively charged ions. The temperature of the ionizing surface and of the tungsten cylinder being at about 600 C. or higher quickly causes the re-evaporation of the positively charged thallium ion from the ionizing surface; these ions are rapidly accelerated through the ion acceleration plate apertures 226 and 228 and into the strong magnetic field of the mass spectrometer 40. The thallium ions are then subsequently deflected by the strong C magnet of the mass spectrometer 40 to a pre: determined and particular arcuate radial path of curvature to effect a mass separation of the thallium ions from ions of a lighter or heavier mass. These ions result from impurities such as potassium ions which has a greater curvature in the same C magnetic field. After passing through the mass spectrometer the thallium ion beam is supplied to the input anode 29 of a multiple stage electron multiplier 30 where the thallium ions are converted to a proportional direct current signal which is fed to the oscillator frequency control circuit.

The tungsten oxide surface 210 has a higher work function than the ionization potential of the thallium. However the abrasion and general deterioration of the oxide contact surface at the elevated temperatures employed would, in continual operation, gradually cause a decline in work function of the surface film to less than the thallium ionization potential as the oxide surface is eroded and decomposed. In the described apparatus and method there exists va slight positive pressure difference between the external sub-atmospheric or vacuum condition outside the tungsten cylinder, i.e. the beam tube vacuum condition of about 1 l0 mm. of mercury or less, and the internal oxygen pressure which may be at atmospheric or higher pressure for example 5 to 30 pounds p.s.i.g. Due to the relatively thin wall thickness between the ionizing sur face 212 and the internal wall surface of the tungsten cylinder the oxygen is slowly diffused through the thin wall section of the oxygen enclosure to continually create, react with and maintain an active tungsten oxide surface on the ionizing surface. This supply of oxygen inhibits the gradual decrease in work function of this surface and enhances the operational life of the ionizer 200.

The diffusion rate of the oxygen through the wall thicknesses is generally responsive to the pressure within the internal portion 204 of the cylinder, the wall thickness, the flow rate from the oxygen source, the temperature of the cylinder, the material of the cylinder and other factors effecting diffusion rates. However the rate of oxygen effusion is generally controlled and adjusted by means of the gas flow control valve 206 to permit suificient oxygen into 1 lthe cylinder 202to maintain a continuous oxide film on the. ionizing surface 208. Higher diffusion rates than necessary are wasteful and tend to reduce the vacuum condition existing within the beam tube. To maintain the molecular beam tube apparatus at a desired vacuum level it is common practice to employ a vacuum creating means such as an ion pump (not shown) in communication with the beam tube at or near the detector end of the beam tube.

Although the ionizer has been described with reference to a particular pressurized source, it is within the scope of my invention that the pressure source may be any diffusible reactive fluid capable of reacting with and maintaining the ionizing surface in a desirable condition. Thus the reactant gas may be any oxidizing or even reducing gas or liquid such as an oxygen-containing gas like steam, air, oxygen, enriched air, or relatively pure oxygen; a halogen like chlorine, bromine, fluorine, iodine, or a halohydrocarbon such as hydrogen chloride, hydrogen bromide and the like; an oxide of sulfur like sulfur trioxide, oxides of nitrogen like nitric oxide, nitrogen dioxide etc. The particular liquid or gas selected depending upon the reaction and the compound desired to be maintained on the ionizing surface. Thus oxides of sulfur would maintain metal sulfate films, oxides of nitrogen for metal nitrate films, halogen for metal halides etc. The reaction illustrated is an oxidation reaction, however diifusible gases may be used in reduction reactions.

Commonly the oxides of elemental metals have a higher work function than the pure elemental metal. Thus, for example, the use of pure elemental tungsten is unsuitable for detecting thallium atoms since it has a work function of only about 4.5 electron volts, while the tungsten oxide has a work function of greater than 6.2 electron volts. Suitable elemental metals whose oxides, halides, sulfides, sulfates, nitrates and the like may be employed for the ionizing surface of the ionizer include but are not limited to: chromium, iridium, nickel, palladium, rhodium, rhenium, platinum, niobium, and other polyvalent heavy metals.

The ionizer of my invention is particularly useful where temperatures of greater than 600 or 1000 C. are required at the ionizing surface to create a sufiiciently high ion evaporation rate. The ionizing surface should be maintained at a sufliciently elevated temperature to permit the rapid reevaporation of the ions on the ionizing surface; otherwise, a relatively cool surface will permit a pile up of the neutral atoms on the surface and result in a decrease in ionization as the neutral atoms cover the ionizing surface. My invention permits the continual maintenance of an oxide film on the ionizing surface under elevated temperature conditions. It thereby permits the use of elevated temperatures of up to 3000 C. in the case of rhenium oxide films, to achieve very high reevaporation rates, without reduction in the work function of the ionizing oxide film. Where the oscillating microwave field is phase modulated for example at 1 to 100 cycles per second the temperature of the ionizing surface should be sufficiently high to permit the dwell time of the particle thereon to be 1 modulation cycle or less.

The enclosure means for the oxygen has been shown as a hollow metal cylinder 202; however, it is recognized that the enclosure may be of varied shape and composed of other than metal providing only that the material permits the desired diffusion of the gas through the wall to the ionizing surface. Further, the ionizing surface and the cylinder may be composed of the same or different metals or materials. Also where the fluid enclosure is not of material suitable for heating by passing an electric current through it, the ionizing surface may be heated by other methods such as by the use of internal electrical heating elements or coils and the like.

Also the fluid introduced into the cylinder can be circulated through the cylinder from one end thereof to the other as long as a slight positive pressure exists between the inside of the cylinder and the ionizing surface.

I have thus described a thallium molecular beam apparatus which is relatively free from C field, inhomogeneity, requires no gettering of expended thallium, has a natural resonance frequency of enhanced accuracy which is over twice the cesium frequency, and has superior operational life. My thallium molecular beam apparatus comprises an improved beam source and collimator which promotes the formation of a beam of desired geometry without danger of condensation of the relatively high melting point thallium. Further my improved wave guide structure incorporates features which inhibit undesired RF propagation in the drift space, permit economical power operation, and operation between offending modes. Furthermore, my improved ionizer permits the conversion of thallium molecules to thallium ions without rapid deterioration of the work function of the ionizing surface. The combination of these improved features with existing features thereby permits the construction of an operational thallium molecular beam apparatus of superior accuracy and long operational life.

What is claimed is:

1. In a molecular beam tube a molecular beam source and collimator which comprises in combination:

container means capable of containing a relatively high melting point molecular beam source material, said container means comprising means forming an elongated aperture to permit the effusion of the said material from the container means; along a predetermined axis;

collimator means coaxially aligned with the aperture means to form the effused material into a molecular beam of predetermined geometric form;

heating means surrounding and disposed in a heat exchange relationship with the container means and the collimator means, said heating means providing sufficient heat to melt the molecular beam source material and create a thermal gradient between the container means and the collimator means, the higher end of the gradient being disposed toward the col-limator means.

2. An apparatus as defined in claim 1 wherein said appar-atus includes and is mounted within jacketing means comprising an inner and outer jacket having a heat insulating sub-atmospheric region enclosed between the inner and outer jacket.

3. An apparatus as defined in claim 1 wherein said heating means is an electric heating element coil with at least a portion thereof disposed peripherally about the exterior of the container means and the collimator means.

4. An elongated tubular molecular beam tube member having disposed at one end thereof the apparatus of claim 1 and at the other end thereof a molecular beam detecting means which tubular element between these means is convoluted to permit adjustment in the path of the molecular beam through the tubular member between the collimator means and the detecting means.

5. An ionizer for converting a beam of neutral atoms into charged ions which ionizer comprises in combination:

a fluid source;

fluid enclosure means having an ionizing surface, said fiuid enclosure means being in a fluid flow communication with the fluid source and adapted to permit the diffusion of the fluid through the enclosure means to an ionizing surface;

a surface ionizing compound on the ionizing surface of the enclosure means capable of converting a beam of neutral atoms to charged ions; and

heating means to maintain the ionizing surface at a predetermined re-evaporation temperature, said surface ionizing compound capable of reacting with the fluid to inhibit the deterioration of the work function of said compound.

6. An apparatus as defined in claim wherein said reactant fluid is an oxygen-containing gas and said ionizing compound is an oxidizable material.

7. An apparatus as 'defined in claim 5 wherein said fluid enclosure means comprises an elongated metal tubular member characterized by having a slot in the outer wall surface thereof of suflicient depth to permit the diffusion of the fluid to the external surface of the slot, which slot external surface comprises the ionizing surface in the beam path of the neutral atoms.

8. An apparatus as defined in claim 5 wherein said fluid enclosure means forms a heating element in an electrical circuit to maintain the ionizing surface at a re-evaporation temperature.

9. An apparatus as defined in claim 5 wherein said fluid flow communication includes fluid flow control means to provide a sutficient fluid to maintain the ionizing compound as a continuous thin film on the ionizing surface at the re-evaporation temperature.

10. An apparatus as defined in claim 5 wherein said fluid enclosure means comprises a heated tubular element containing tungsten characterized by an ionizing compound of tungsten oxide and wherein the fluid is an oxygen-containing gas which gas is permitted to diffuse to the ionizing surface to maintain a film of tungsten oxide on the ionizing surface capable of converting thallium molecules to charged ions.

11. An ionizer for converting a beam of neutral thallium atoms to positively charged ions which ionizer comprises in combination:

a pressurized oxygen-containing gaseous source;

an enclosure means in gas flow communication with the oxygen-containing gaseous source, said enclosure means characterized by an oxidizable ionizing surface having a higher work function than thallium and adapted to permit the slow difiusion 'of the oxygen-containing gas through the wall of the enclosure means to the ionizing surface to maintain an oxide ionizing film surface for the conversion of neutral thallium atoms to positively charged ions;

and heating means to maintain the ionizing surface at a predetermined re-evaporation temperature of at least 600 C.

12. An ionizer as described in claim 11 wherein said gas source is oxygen and said ionizing oxidizable surface is tungsten.

'13. An elongated evacuated molecular beam tube member having disposed at one end thereof a source'of thallium capable of passing in beam form from the one end to the other end and at the other end thereof a thallium ionizer as described in claim 11 to convert the thallium molecules into thallium ions.

14. An improved microwave guide structure which comprises in combination; a U-shaped microwave means characterized by a center microwave feed point and having disposed at equal electrical lengths from the said feed point at least one resonant cavity having an exit and entrance beam windows at either end of the resonant cavity said beam windows composed of a plurality of relatively small diameter longitudinally tubular members disposed about the axial line of the cavities in sufficient amount and nature to inhibit the propagation of microwaves from the cavity.

15. An improved microwave guide structure as set forth in claim 13 wherein said structure comprises two resonant cavities having entrance and exit beam windows said cavities spatially separated in an axially line to permit the passage therethrough of a molecular beam, each cavity being equidistant from the center feed point.

16. A thallium molecular beam apparatus which employs a change in energy state between molecules to derive a frequency standard signal said apparatus comprising in combination:

a source of thallium;

detector means to detect and proportionally convert the beam of thallium atoms into an electrical signal;

an elongated evacuated molecular beam tube having axially disposed at one end thereof the thallium source and at the other end thereof the detector means;

heated collimator means adapted to form thallium from the source into a thallium molecular beam of predetermined geometric form;

A and B energy state selecting means to select the desired energy states of the thallium as it passes through the beam tube;

means to create a weak uniform magnetic field between the A and B energy state selector means and in which field is located resonant cavity means through which the thallium beam passes and in which the thallium beam means is subjected to an intense oscillating field of predetermined frequency and intensity to effect the desired energy state transitions.

17. A thallium molecular beam apparatus defined in claim 16 wherein said thallium source is a heated thallium oven and in which apparatus collimator means is axially aligned with and adjacent to said oven, said collimating means in a heat exchange relationship with the oven to inhibit the condensation of the thallium within the coll-imator means.

18. A molecular beam apparatus as defined in claim 16 wherein said resonant cavity means comprises a waveguide structure having two resonant cavities in axially spaced relationship at equal electrical lengths from a center microwave feed point, each cavity disposed along the axially path of the thallium beam and characterized by an entrance and exit beam windows through which the beam is projected, each window comprising a plurality of relatively small diameter longitudinal tubular member to inhibit RF propagation in the drift space between the cavities.

19. A thallium molecular beam apparatus defined in claim 16 wherein said detector means comprises a thallium ionizing means having an ionizing surface in the thallium beam path, the ionizing surface being maintained at a work function higher than the ionization potential of thallium by the diflfusion of an oxygen-containing gas to the ionizing surface; mass separation means to separate the positively charged thallium ions from the undesired charged particles, and electron multiplying means to convert the ionized thallium beam into an electrical signal.

References Cited UNITED STATES PATENTS 2,972,115 2/1961 Zacharias et a1 331- 3 2,991,389 7/1961 Grant et a1 3-31 3 x 3,060,385 10/1962 Lipps 6: al. 331-3 3,243,640 3/1966 Eichenbaum 31s 3.s

OTHER REFERENCES Buckingham et al.: Mercury Thallium and Lead Resonances, Faraday Society Discussions No. 33-35, T No. 34, 196 2, pages 147 to 155. I

HERMAN KARL SAALBACH, Primary Examiner. S. CHATMON, JR., Assistant Examiner. 

1. IN A MOLECULAR BEAM TUBE A MOLECULAR BEAM SOURCE AND COLLIMATOR WHICH COMPRISES IN COMBINATION: CONTAINER MEANS CAPABLE OF CONTAINING A RELATIVELY HIGH MELTING POINT MOLECULAR BEAM SOURCE MATERIAL, SAID CONTAINER MEANS COMPRISING MEANS FORMING AN ELONGATED APERTURE TO PERMIT THE EFFUSION OF THE SAID MATERIAL FROM THE CONTAINER MEANS; ALONG A PREDETERMINED AXIS COLLIMATOR MEANS COAXIALLY ALIGNED WITH THE APERTURE MEANS TO FORM THE EFFUSED MATERIAL INTO A MOLECULAR BEAM OF PREDETERMINED GEOMETRIC FORM; HEATING MEANS SURROUNDING AND DISPOSED IN A HEAT EXCHANGE RELATIONSHIP WITH THE CONTAINER MEANS AND THE COLLIMATOR MEANS, SAID HEATING MEANS PROVIDING SUFFICIENT HEAT TO MELT THE MOLECULAR BEAM SOURCE MATERIAL AND CREATE A THERMAL GRADIENT BETWEEN THE CONTAINER MEANS AND THE COLLIMATOR MEANS, THE HIGHER END OF THE GRADIENT BEING DISPOSED TOWARD THE COLLIMATOR MEANS.
 16. A THALLIUM MOLECULAR BEAM APPARATUS WHICH EMPLOYS A CHANGE IN ENERGY STATE BETWEEN MOLECULES TO DERIVE A FREQUENCY STANDARD SIGNAL SAID APPARATUS COMPRISING IN COMBINATION: A SOURCE OF THALLIUM; DETECTOR MEANS TO DETECT AND PROPORTIONALLY CONVERT THE BEAM OF THALLIUM ATOMS INTO AN ELECTRICAL SIGNAL; AN ELONGATED EVACUATED MOLECULAR BEAM TUBE HAVING AXIALLY DISPOSED AT ONE END THEREOF THE THALLIUM SOURCE AND AT THE OTHER END THEREOF THE DETECTOR MEANS; HEATED COLLIMATOR MEANS ADAPTED TO FORM THALLIUM FROM THE SOURCE INTO A THALLIUM MOLECULAR BEAM OF PREDETERMINED GEOMETRIC FORM; A AND B ENERGY STATES SELECTING MEANS TO SELECT THE DESIRED ENERGY STATES OF THE THALLIUM AS IT PASSES THROUGH THE BEAM TUBE; MEANS TO CREATE A WEAK UNIFORM MAGNETIC FIELD BETWEEN THE A AND B ENERGY STATE SELECTOR MEANS AND IN WHICH FIELD IS LOCATED RESONANT CAVITY MEANS THROUGH WHICH THE THALLIUM BEAM PASSES AND IN WHICH THE THALLIUM BEAM MEANS IS SUBJECTED TO AN INTENSE OSCILLATING FIELD OF PREDETERMINED FREQUENCY AND INTENSITY TO EFFECT THE DESIRED ENERGY STATE TRANSITIONS. 